This invention relates to a system and method for controlling a transfer case and more particularly, to a system and method for controlling a transfer case within a four-wheel drive vehicle having variations in tire diameter, such as those caused by tire wear and inflation, and which provides an improved response by automatically adjusting to the condition of the vehicle""s tires.
Four-wheel drive vehicles typically include a transfer case which selectively transfers torque and power from the vehicle""s input shaft to a front and a rear driveshaft, thereby selectively rotating the front and rear driveshafts. The transfer case includes a conventional electromagnetic clutch assembly which typically and selectively transfers torque from a primary driveshaft (e.g., the rear driveshaft) to a secondary driveshaft (e.g., the front driveshaft), thereby reducing the torque provided to the primary driveshaft and increasing the torque provided to the secondary driveshaft. The clutch assembly is typically and communicatively coupled to a controller and/or a control system which determines the amount of torque that is to be provided to the front and rear driveshafts based upon vehicle sensory data, and which generates a control or command signal to the clutch assembly, thereby controlling the amount of torque which is provided to the front and rear driveshafts.
In order to provide torque and traction to the front and rear wheels of the vehicle and to avoid relative slip between the front and rear wheels, the transfer case control system typically monitors and adjusts the speed of both the front and rear driveshafts. For example and without limitation, when the speed of the front wheels differs from the speed of the rear wheels by a certain predetermined amount or xe2x80x9cerrorxe2x80x9d, these prior systems alter or change the value of the control or command signal to provide more or less torque to the front and/or rear driveshaft in order to prevent relative slip between the front and rear wheels. Particularly, these prior systems typically alter the magnitude of the control signal when the difference between the speed of the front and rear driveshafts (e.g., the xe2x80x9cdelta shaft speedxe2x80x9d) exceeds an xe2x80x9callowable slipxe2x80x9d factor or value.
The xe2x80x9callowable slipxe2x80x9d factor is utilized in order to account for variations in tire wear and inflation, which can cause a difference in the speed of the front and rear axles and driveshafts by changing the effective rolling radius of the individual tires. Since this type of speed difference results xe2x80x9cnaturallyxe2x80x9d from the physical attributes of the tires and the tire/road interface, and not from relative slip between the front and rear wheels of the vehicle, the xe2x80x9callowable slipxe2x80x9d factor or value is necessary to prevent the control system from constantly activating or attempting to prevent the tires from rotating at their xe2x80x9cnaturalxe2x80x9d speed. Unnecessary and repeated activation of the control system caused by variations in tire wear and inflation could cause noise and vibration harshness (xe2x80x9cNVHxe2x80x9d) problems and unnecessary wear of the tires, clutch assembly, and other drivetrain components.
For these reasons, prior transfer case control systems typically set or establish a relatively xe2x80x9cgenerousxe2x80x9d or large allowable slip factor in order to account for the xe2x80x9cworstxe2x80x9d or largest possible wheel speed difference that could occur xe2x80x9cnaturallyxe2x80x9d as a result of variations in tire wear and inflation. This allowable slip factor or value typically varies based upon the speed of the vehicle, and is determined or calculated by way of a table or matrix stored within the memory of the system. The allowable slip factor or value also typically varies based upon whether the front or the rear wheels of the vehicle are slipping (e.g., prior systems typically include a rear slip table having allowable xe2x80x9crear slip valuesxe2x80x9d and a front slip table having allowable xe2x80x9cfront slip valuesxe2x80x9d).
While these prior systems do reduce the likelihood of unnecessary activation and/or cycling of the transfer case in response to variations in the effective diameter of the tires, they suffer from some drawbacks. Particularly, because the allowable slip factors or values utilized by these systems are relatively large or xe2x80x9cgenerousxe2x80x9d, when a true slip event occurs, a relatively high degree of slip must be achieved before the controller detects or determines that a slip condition has occurred which is not attributable to variations in tire wear and inflation. For example and without limitation, if a vehicle having substantially matching or equal tire wear and inflation is being driven at 100 kilometers per hour (xe2x80x9ckphxe2x80x9d), and the allowable rear slip factor or value is 3 kph, the speed of the rear wheels would have to exceed 103 kph, which may represent a relatively severe slip condition, before a slip control would be generated effective to transfer torque to the front wheels of the vehicle. Hence, the overall torque adjustment xe2x80x9cresponsexe2x80x9d of these prior systems is adversely effected due to these xe2x80x9cgenerousxe2x80x9d slip factors which delay the systems"" ability to respond until a relatively high degree of slip has occurred.
There is therefore a need for a system and method for controlling a transfer case within a four-wheel drive vehicle which provides an improved response by automatically adjusting to the condition of the vehicle""s tires.
A first advantage of the invention is that it provides a system and method for controlling a transfer case within a four-wheel drive vehicle which overcomes at least some of the previously delineated drawbacks of the prior systems, devices, and/or methods.
A second advantage of the invention is that it provides a system and method for controlling a transfer case within a four-wheel drive vehicle which monitors relative slip between the front and rear wheels of the vehicle and which provides a relatively quick response to relative slip situations.
A third advantage of the invention is that it provides a system and method for controlling a transfer case within a four-wheel drive vehicle which automatically generates xe2x80x9cless generousxe2x80x9d or xe2x80x9ctighterxe2x80x9d allowable slip values based upon the condition of the vehicle""s tires.
According to one aspect of the present invention, a system for controlling a transfer case in a vehicle is provided. The vehicle is of the type having a front driveshaft which rotatably drives a pair of front wheels having a first effective diameter at a first speed, and a rear driveshaft which rotatably drives a pair of rear wheels having a second effective diameter at a second speed. The transfer case is effective to selectively transfer torque to the front driveshaft and to the rear driveshaft. The system includes a first sensor which measures the first speed and which generates a first signal which represents the first speed; a second sensor which measures the second speed and which generates a second signal which represents the second speed; and a controller which is communicatively coupled to the transfer case and to the first sensor and the second sensor. The controller receives the first signal and the second signal, and calculates a difference value between the first speed and the second speed. The controller is adapted to selectively generate a slip control signal when the difference value exceeds an allowable slip value. The controller is further adapted to selectively alter the slip value based upon the first effective diameter and the second effective diameter.
According to a second aspect of the present invention, a method for controlling a transfer case is provided. The transfer case is operatively disposed within a four wheel drive vehicle of the type having a pair of front wheels having a first effective diameter and a pair of rear wheels having a second effective diameter, a front driveshaft which is effective to rotate the front wheels at a first speed, and a rear driveshaft which is effective to rotate the rear wheels at a second speed. The transfer case is adapted to selectively provide torque to the front driveshaft and the rear driveshaft. The method includes the steps of: selectively generating a first and a second allowable rear slip value; determining whether a difference exists between the first and the second effective diameters; selecting a unique one of the first and the second allowable rear slip values based upon the determination; measuring the first speed and the second speed; and selectively generating a first signal if the second speed exceeds the first speed by a margin greater than the selected unique one of the first and the second allowable rear slip values, the first signal being effective to control the torque which is provided to the front and the rear driveshafts.
Further objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred embodiment of the invention and by reference to the following drawings.